The present invention relates to thermoplastic elastomeric compositions obtained by mixing and dynamically cross-linking a thermoplastic olefin polymer, which comprises an unsaturated polyolefin, and an unsaturated elastomeric olefin polymer.
Thermoplastic elastomeric compositions obtained by mixing in the molten state a thermoplastic olefin polymer with an unsaturated elastomeric olefin polymer (such as an ethylene-propylene-diene rubber, known as EPDM) and dynamic cross-linking, such as the ones described in published European patent application 213 285 in the name of the Applicant, are known in the art. By dynamic cross-linking one means that the crows-linking agent is added to the polymers described above before or during mixing, and said mixing is continued in the molten state during the cross-linking stage. The practical interest of compositions thus obtained is due to the fact that, although they can be processed and molded with apparatus and processes typical of thermoplastic polymers, they supply products which have elastomeric characteristics that, without being high, are in fact satisfactory for many application sectors, such as the construction, automotive and appliance industries.
Now the Applicant has obtained some thermoplastic elasomeric olefin compositions that, while maintaining good processability, possess improved elastic characteristics, as can be seen by the particularly low compression set values.
Therefore, the present invention provides thermoplastic elastomeric compositions comprising:
A) from 10 to 80% by weight, preferably from 20 to 60%, of one or more crystalline polymer of propylene containing from 0.05 to 15% in moles, preferably from 0.1 to 10%, of ethylenic unsaturation (unsaturated polymer), or a mixture of the above polymer with a saturated thermoplastic olefin polymer in quantities lower than or equal to 80% by weight;
B) from 20 to 90% by weight, preferably from to 80%, of an unsaturated elastomeric olefin polymer;
said compositions having been dynamically cross-linked with a cross-linking agent comprising a free radical generator.
Examples of unsaturated polymers that can be used as component (A) are isotactic, or mostly isotactic, propylene polymers, preferably having an isotactic index in xylene at 20xc2x0 C. greater than 70%, and copolymers of propylene with ethylene and/or C4-C10 xcex1-olefins. The ethylene and/or C4-C10 xcex1-olefin content in the copolymers is preferably from 1 to 30% by weight, more preferably from 1 to 20%.
Examples of C4-C10 xcex1-olefins are (1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene and 6,6-dimethyl-1-pentene.
The ethylene unsaturation in the above polymers can be introduced directly in synthesis, copolymerizing minor quantities of a conjugated or nonconjugated C4-C10 diene. Examples of the above dienes are: 1,3-butadiene, isoprene, 1,3-pentadiene, 1,4-hexadiene, dicyclopentadiene and 2-ethylidene-5-norbornene.
The synthesis of the polymers containing the unsaturation is carried out by known polymerization techniques with coordination catalysts, operating in liquid phase, or gas phase, or a liquid-gas phase. The catalysts used are coordination catalysts known in the art, preferably high yield and high stereospecificity ones, comprising a titanium compound supported on a magnesium halide, in particular MgCl2.
In order to obtain the molar quantities oft ethylenic unsaturation mentioned above, one copolymerizes the same molar quantities of the above mentioned dienes.
Examples of the above mentioned polymerization processes are described in published European patent application n. 171 025, in the name of the Applicant. As explained in said European patent application, the conjugated dienes can be bonded to the polymer chains in position 1,2 or 1,4, so that the ethylenic unsaturation is present along the chain (position 1,4) or in branched groups (position 1,2). By using the coordination catalysts mentioned above, one generally obtains introductions of the conjugated diene both in 1,2 position and 1,4 position.
Also included within the definition of the present invention are the thermoplastic elastomeric compositions wherein part of the unsaturated polymer constituting component (A) is substituted by a saturated thermoplastic olefin polymer. This way one obtains a mixture of a saturated and an unsaturated polymer. Examples of the above mentioned saturated polymers are the ethylene homopolymers, and the crystalline copolymers of ethylene with xcex1-olefins, such as HDPE, LLDPE, LDPE, or with other nonhydrocarbon monomers, such as vinyl acetate, acrylic acid, methacrylic acid, acrylic and met acrylic esters, maleic anhydride, or homopolymers and copolymers of the types previously defined for component (A), where no ethylenic unsaturation is introduced.
In order to obtain the best elastic properties in the final compositions, it is best if the saturated polymers do not constitute over 50% by weight of component (A) in the formulations defined above.
Examples of elastomeric olefin polymers that can be used as component (B) are the mainly amorphous terpolymers of ethylene, containing from 20 to 80% by weight of propylene and or C4-C10 xcex1-olefins, and from 0.5 to 15% by weight of a conjugated or nonconjugated C4-C10 diene. Examples of xcex1-olefins and C4-C10 dienes are those already given for component (A). Particularly preferred elastomeric polymers are the ethylene/propylene/2-ethylidene-5-norbornene terpolymers containing from 25 to 70% by weight of propylene, and from 4 to 10% by weight of 2-ethylene-5-norbornene.
The compositions of the present invention are prepared by a process comprising a dynamic cross-linking step consisting of subjecting components (A) and (B) to mixing at a temperature higher than or equal to their softening or melting point, in the presence of the cross-linking agent, that can be added before, during or after the first mixing step, continuing said mixing throughout the cross-linking step. During the mixing step one obtains a fine dispersion of component (B) in component (A). Preferably, component (B) is dispersed in the form of particles having a diameter ranging from 0.5 to 1 micrometer. The cross-linking agent can be added before, during, or after the above mentioned mixing stage. The mixing can occur in an internal mixer, in an extruder, or in a system comprising an internal mixer and an extruder. The operation can also take place by using a number of machines placed in series, where the intimate mixing and homogenization of the compositions occurs in the first machine, and the cross-linking occurs in the others, always under mixing conditions. Mineral fillers, carbon black, dyes, plastifiers, stabilizing agents, extender oils, and other conventional additives which are typically used in unsaturated elastomeric olefin polymer compositions, can be present in the composition that is subjected to mixing and dynamic cross-linking.
The cross-linking step is carried out at temperatures preferably ranging from 170 to 230xc2x0 C., by way of adding a free radical generator preferably consisting of an organic peroxide.
The free radical generator is preferably added in quantities ranging from 0.1 to 10% by weight, more preferably from 1% to 5% by weight with respect to the sum of components (A) and (B).
Examples of peroxides that can be used as free radical generators are: 1,1-bis(tert-butylperoxy)3,5,5-trimethyl cyclohexane; tert-butylperbenzoate; 2,2-bis(tert-butylperoxy)butane; dicumyl peroxide; di-tert-amyl peroxide; di-tert-butyl peroxide; 1,3-bis(tert-butylperoxy isopropyl)benzene. The cross-linking action can also be supported by the addition of cross-linking coagents, such as: trialyll cyanurate; triallyl isocyanurate; maleimides; allyl acrylates; divinylbenzene; polybutadiene, and benzoquinone dioxime.
Particularly indicated for this purpose are some unsaturated furfuryl aldehyde condensation products with hydrazine, acrolein; acetone, acrylamide or acrylonitrile, such as: i-difurfuralaldazine; 3(xcex1-furyl)acrolein; 1,5-difurfuryl-1,4-pentadiene-3-one; 3-(xcex1-furyl)acrylamide; 3-(xcex1-furyl)acrylonitrile. The preferred compound is the difurfuralaldazine.
Preferably, the cross-linking coagents are added in quantities ranging from 5% to 60% by weight with respect to the free radical generator.